Preheat oven to 350 degrees.
Drain cooked spinach in colander. When cool enough to handle, squeeze out as much water as possible. Chop and set aside.
Heat oil and sauté onions and scallions until soft but not brown, about 3 minutes.
Add spinach, parsley, and dill, and cook, stirring until spinach has wilted, about 2-3 minutes.
Remove from heat, transfer to a bowl and let cool.
In a separate bowl, lightly beat egg whites. Add cheeses and chopped olives and blend well.
Combine egg-cheese mixture with spinach.
Slice open the top half of the small pitas, or cut the large pitas in half.
Stuff spinach mixture into pitas and place in a loaf pan or casserole dish, standing upright as much as possible, leaning against one another.
Cover with foil and bake until set and heated through, about 30 minutes.
Let cool slightly before eating.